A major component of the development of a hydrogen economy is the wide scale adoption of fuel cell technology. While there have been significant advances towards the application of fuel cells in everyday life, their widespread use has not been achieved yet due in part to the high cost of electricity they produce, see Rose, R., Fuel Cells and Hydrogen: The Path Forward, Report Prepared for the Senate of the USA, <http://www.fuelcellpath.org>.
The slow kinetics of the oxygen reduction reaction on the cathode of the most popular proton-exchange membrane (PEM) hydrogen-oxygen fuel cell is the main reason for both the high cost of the fuel cell itself (requirement of Pt as catalyst) and of low electrical fuel efficiency, around 50% as disclosed in Bockris, J. O.-M. and R. Abdu, J. Electroanal. Chem., 448, 189 (1997).
The use of redox fuel cells, in which oxygen is replaced by other oxidants, such as ferric ions, can result in the increase of the rate of cathodic reaction (or exchange current density in electrochemical terms) by several orders of magnitude, as disclosed in Bergens, S. H., G. B. Gorman, G. T. R. Palmore and G. M. Whitesides, Science, 265, 1418 (1994); Larsson, R. and B. Folkesson, J. Appl. Electrochem., 20, 907 (1990); and Kummer, J. T. and D.-G. Oei, J. Appl. Electrochem., 15, 619 (1985).
In addition, the rate of mass transfer of oxidant to the electrode surface (corresponding to limiting current density in electrochemical terms) is also higher, mainly because of the higher aqueous solubility of the oxidant in redox fuel cells (for example, 50 g/L for Fe3+) as compared to that of oxygen (between 0.006 and 0.04 g/L, depending on the partial pressure and temperature). All these characteristics of the redox fuel cells should theoretically allow efficiencies for the transformation of chemical to electrical energy of 80 to 90% to be achieved using non-noble metal electrodes based on thermodynamic arguments. However, the main problem in redox fuel cells is the efficiency of reoxidation of the reduced form of the oxidant (oxidant regeneration), see Larsson, R. and B. Folkesson, J. Appl. Electrochem., 20, 907 (1990); and Kummer, J. T. and D.-G. Oei, J. Appl. Electrochem., 15, 619 (1985).
For example, γ-ray irradiation has been used for the reoxidation of Fe2+ to Fe3+ in a H2—Fe3+/Fe2+ redox fuel cell as disclosed in Yearger, J. F, R. J. Bennett and D. R. Allenson, Proc. Ann. Power Sources Conf., 16, 39 (1962). While the efficiency of the fuel cell itself was very high, the reported efficiency of the oxidant regeneration was well below 15%. In other cases, regeneration of the oxidant is carried out using oxygen over expensive catalyst [see Bergens, S. H., G. B. Gorman, G. T. R. Palmore and G. M. Whitesides, Science, 265, 1418 (1994)] which eliminates the advantage of the use of non-platinum cathode, and is still slow.
Therefore, in order to develop a practically viable redox fuel cell with high overall efficiency, it is necessary to develop an efficient method for oxidant regeneration as suggested in Larsson, R. and B. Folkesson, J. Appl. Electrochem., 20, 907 (1990).
The process of aerobic oxidation of ferrous to ferric ions by chemolithoautotrophic microorganisms such as Acidithiobacillus ferroxidans (A. ferrooxidans) was discovered more than half a century ago, see A. R. Colmer, M. E. Hinkle, Science, 106 (1947) 253-256. These microorganisms have been widely used in metallurgy for the leaching of noble (Au), heavy (U) and base (Cu, Ni, Zn, Co) metals, as well as in environmental protection. The microbial iron oxidation is based on the following net reaction:4Fe2++4H++O2=4Fe3++2H2O  (1)
It has been shown that the rate of microbial oxidation of ferrous ions is 500,000 times faster than that obtained by purely chemical reaction with oxygen at pH between 1 and 2, see D. T. Lacey, F. Lawson, Biotechnology and Bioengineering, 12 (1970) 29-50.
When growing on ferrous iron oxidation, A. ferrooxidans uses one of the narrowest thermodynamic limits known in microbial world, see W. J. Ingledew, Biochimica et Biophysica Acta, 683 (1982) 89-117. The electron transport chain of iron oxidation by this microorganism contains two half-reactions:4Fe2+=4Fe3++4e−  (2)which takes place outside of the cell membrane, and4e−+O2+4H+=2H2O  (3)inside of the membrane, see M. Nemati, S. T. L. Harrison, G. S. Hansford, C. Webb, Biochemical Engineering Journal, 1 (1998) 171-190. The electrons are transported through the cell wall via a chain of three electron carriers: rusticyanin, cytochrome c and cytochrome a.
The iron-oxidizing bacterium A. ferrooxidans is an autotrophic microorganism, i.e. it uses carbon dioxide (CO2), usually from atmosphere, as a sole source of carbon, while inorganic reactions such as ferrous iron oxidation (1-3) supply it with energy. The laboratory-, pilot-, and industrial-scale oxidation of iron by A. ferrooxidans has been studied in different types of bioreactors. Under the usual cultivation conditions in a bioreactor containing A. ferrooxidans grown on ferrous ions, the redox potential can reach a value of 1000 mV, see M. Boon, K. C. A. M. Luyben, J. J. Heijnen, Hydrometallurgy, 48 (1998) 1-26. Since the potential of reaction (3) is 1120 mV vs. standard hydrogen electrode (SHE), up to approx. 90% of the reaction energy is used for the production of Fe3+, while the rest (˜10%) is available to microorganisms for biomass formation and maintenance.
The biooxidation of ferrous iron by A. ferrooxidans has been used in electrochemical cells for several different purposes. In all these cases, the electrochemical reaction, taking place on the surface of the cathode is:Fe3++e−=Fe2+  (4)Several different counter-electrode (anode) reactions have been described:A) Oxygen Formation According to the Reaction:2H2O=4e−+O2+4H+  (5a)In that case, it is necessary to apply external electrical potential in order to reduce the ferric iron on one electrode and to produce oxygen on the other. This system has been used for the continuous regeneration of the microbial substrate (ferrous iron) which resulted in the production of very high cell yields, see N. Matsumoto, S. Nakasono, N. Ohmura, H. Saiki, Biotechnology and Bioengineering, 64 (1999) 716-721; and S. B. Yunker, J. M. Radovich, Biotechnology and Bioengineering, 28 (1986) 1867-1875.B) Oxidation of Ferric Ions:Fe2+=Fe3++e−  (5b)This type of electrobioreactor has been used to determine the rate of microbial ferrous iron oxidation by measuring the value of the electrical current, see H. P. Bennetto, D. K. Ewart, A. M. Nobar, I. Sanderson, Charge Field Eff. Biosyst.—2, [Proc. Int. Symp.], (1989) 339-349; and K. Kobayashi, K. Ibi, T. Sawada, Bioelectrochemistry and Bioenergetics, 39 (1996) 83-88.C) Oxidation of Organic Compounds such as Methanol:CH3OH+H2O=CO2+6H++6e−  (5c)This system has been used for the electrochemical degradation of pollutants (methanol) in water, see A. Lopez-Lopez, E. Exposito, J. Anton, F. Rodriguez-Valera, A. Aldaz, Biotechnology and Bioengineering, 63 (1999) 79-86.
The above analysis of the energetics of ferrous iron oxidation by A. ferrooxidans shows that up to 90% of the Gibbs energy of microbial oxygen reduction can be used for the iron oxidation, i.e. production of electricity, while the rest will be consumed by the microorganisms for maintenance and formation of new cell biomass. It has also been found that the growth of A. ferrooxidans can be uncoupled from iron oxidation under certain conditions, see M. Nemati, S. T. L. Harrison, G. S. Hansford, C. Webb, Biochemical Engineering Journal, 1 (1998) 171-190, i.e. these microorganisms can oxidize ferrous iron under zero-growth conditions.
It has been recognized that the global warming, caused mainly by anthropogenic carbon dioxide emissions, is one of the main problems which humanity faces at the moment. Presently, the most promising way to reduce the release of carbon dioxide to atmosphere seems to be the transition from fossil fuel economy to hydrogen economy, see J. O. M. Bockris, International Journal of Hydrogen Energy, 27 (2002) 731-740.
Presently known oxygen/hydrogen fuel cells do not produce carbon dioxide when using hydrogen as fuel. However, it would be even more advantageous to provide a bio-fuel cell based on chemolithoautotrophic microorganisms which exhibit very high efficiency for production of electricity and which consumes CO2 from atmosphere during its operation.